I'm not quite sure how I'd use the metric or if I'd use it to purge notes.
Searchability or recall can be a problem sometimes though, so "searches where I had no results or didn't visit anything" could be interesting.
Especially if I try searching later and find a note answering my question with bad "SEO".
Another idea I had was to make a Firefox extension that searches my notes and displays results before search engines since I reflexively search things in browser sometimes.
I just, again, for me personally, gain a lot of value from writing the notes even that I never go back to revisit.
So for me personally, the metric is interesting, like I said, but doesn't capture "how useful" a note is, because I have most of my utility outside of that use case.
1. I have it committed to memory. 2. I never needed it.
I have no way to discern whether either (1) or (2) will happen as far into the future as you care to specify; so it's mostly a moot point. In any case, I will sometimes just do random walks through my notes, wikipedia style, and find a lot of value in it.
While I think it's an interesting metric, it wouldn't capture the utility of my notes for me (emphasis on "for me", since everyone's probably different when it comes to notes).
Often, the act of writing the note helps better commit what I'm writing to memory. At a super rough estimate I'd say that 80% of the utility of note-taking is the act of producing the note itself, and only 20% of the utility is being able to refer back to specific facts.